The present invention relates to an exhaust gas purifying catalyst.
As a catalyst metal included in an exhaust gas purifying catalyst for cars, a noble metal of Rh is widely used because of its high performance for reduction purifying NOx included in exhaust gas.
Japanese Laid-Open Patent Publication No. 11-138001 discloses an exhaust gas purifying catalyst in which Pt and Rh are both carried on a particulate zirconium-based composite oxide and a heat-resistant supporting carrier is coated with the zirconium-based composite oxide together with at least an oxygen storage rare earth oxide. This publication describes that since both Pt and Rh are carried on the zirconium-based composite oxide in this exhaust gas purifying catalyst, high catalytic activity can be expected even under severe high temperature circumstances.
Japanese Patent Publication No. 5-47263 discloses an exhaust gas purifying catalyst composed of a honeycomb carrier coated with a catalyst composition including platinum group metal carrying zirconia in which a platinum group metal including Rh is carried on zirconia powder and further including a refractory inorganic oxide and a rare earth oxide. This publication describes that the catalyst in which the platinum group metal is carried on the fine zirconia particles exhibits high initial performance and has high durability even under severe endurance conditions like a high temperature oxidizing atmosphere.
In an exhaust gas purifying catalyst, in order to attain a large contact area with exhaust gas for achieving high purifying performance, a material for carrying a catalyst metal is generally one having a large specific surface area, which is typically Al2O3.
In the case where Rh is carried on Al2O3 as the catalyst metal, however, solid solution of the Rh into the Al2O3 is proceeded through repeated uses at a high temperature over a long period of time, resulting in lowering the catalytic activity with time. Therefore, Rh is carried on a ceria material. When a ceria material is thermally aged through repeated uses at a high temperature, however, its specific surface area is largely reduced through sintering as compared with that of Al2O3, and therefore, it is necessary to increase the amount of Rh to be carried in order to attain high purifying performance.